This invention relates to a surface mount for an electrical component and more particularly to a surface mount for a metal oxide varistor.
Surface mounting of components on printed circuit boards is common and it is desirable that the mounts be low in profile so that the distance between boards can be minimized and the size of the electronic chassis thus reduced. Where the electrical component is a thin element such as a disc with electrodes on both flat sides, low profile mounts are particularly desirable.
It is also desirable that the number of components required for mounting be minimized because of the area on the circuit board each component occupies. Thus it is highly desirable that the mount be compatible with components having multiple electrodes.
The thermal characteristics of the electrical components is generally a concern and it is desirable that the surface mount be compatible with, or integrated with one or more thermally sensitive switches. The thermal characteristics for many electrical circuit components such as varistors are particularly important where the component is coated with epoxy. In the event of a sustained overvoltage condition across the component, the current through the component can cause the temperature to increase sufficiently to cause the epoxy coating to smoke or ignite. Similarly, the epoxy coating can smoke or ignite in the event the component is subjected to a significant current pulse such that the component internally shorts but does not destruct, i.e., does not open-circuit.
The current solution to this overheating problem is to wire in series with the component a thermal cutout device in sufficiently close physical proximity to respond to the heat in the component to short circuit and thereby disconnect the component. This solution requires the addition of extra components and increases the cost and complexity of the circuit. Since great care must be taken to keep the thermal cutout device in close physical proximity to the component to preserve the responsiveness without overreacting, interference may occur with any surface mount.
Additionally, the "thermal connection" between the component and the thermal cutout device may be difficult to optimize in a surface mount.
Finally, low temperature thermal cutout devices may not be able to survive the soldering operation required to attach the component to a printed circuit board or the like, and a surface mount to which the component may be mounted without soldering is highly desirable.
Many of the above problems are exacerbated where the electrical component is a disc varistor. Disc varistors are well known and generally comprise a thin disc of a metal oxide or other voltage variable resistive material with an electrode on opposite flat sides of the varistor material. Known surface mounts for the smaller sizes, e.g., 7-10 mm, generally include a molded plastic body into which spring electrodes exiting the sides of the body are internally exposed in position to be contacted by the electrodes of the disc varistor when inserted therebetween from the front of the body.
One example of such prior art mount is illustrated in FIGS. 1 and 2 where the disc varistor 18 comprises a disc of varistor material 20 with surface electrodes 22 and 24 on the flat sides thereof. The varistor 18 may be inserted into the front opening of a plastic body 26 which carries a top contact 28 and a bottom contact 30 conveniently spring biased toward each other and separated by the insertion of the disc varistor. The ends of the contacts 32, 34 may be used to electrically connect the circuit to other components on the printed circuit board on which the body 26 may be mounted. Such mounts are expensive to manufacture and are generally higher in profile than desired. Moreover, they do not provide for multiple electrodes, for temperature protection or for a fuse.
Accordingly, it is an object of the present invention to obviate many of the above problems and to provide a novel surface mount and method for an electrical component on a printed circuit board.
It is another object of the present invention to provide a novel surface mount and method in which the mount is low profile.
It is still another object of the present invention to provide a novel surface mount and method in which the component may be connected without soldering.
It is yet another object of the present invention to provide a novel surface mount and method which is readily adapted for multiple electrical connections.
It is yet still another object of the present invention to provide a novel surface mount and method which is compatible with a thermal switch.
It is a further object of the present invention to provide a novel surface mount which is inexpensive and simple in construction.
It is yet a further object of the present invention to provide a novel surface mount and method which short circuit protection is available.
It is still a further object of the present invention to provide a novel surface mount and method in which the mount may be coated with a moisture barrier and/or have edge passivation.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.